The present invention relates to the bleaching of lignocellulosic material in the form of chemical pulp. More particularly, the present invention relates to the final bleaching of the pulp by using chlorine dioxide and hydrogen peroxide as bleaching chemicals.
In order to be able to carry out peroxide bleaching effectively, the pulp must be substantially free of metal ions, especially manganese ions. The metal ions are normally dissolved out in the chlorine dioxide step as a consequence of the conditions in this step. The metal ions are then removed in subsequent washing steps, whereafter the pulp can be peroxide bleached. In most cases this is sufficient to bring about a satisfactory ISO-brightness with a reasonable peroxide consumption
In accordance with the present invention, it has now been found possible, however, to further lower the metal ion content in the pulp, and to therefore reduce the peroxide consumption and/or to increase the ISO-brightness thereof. According to the present invention, this is achieved by treating the pulp with a chelating agent in direct connection with the chlorine dioxide step.
In accordance with the present invention, it has now been discovered that these objects can now be accomplished by the invention of a method for bleaching a chemical pulp comprising a final bleaching sequence including bleaching the chemical pulp with chlorine dioxide, treating the chemical pulp with a chelating agent, the bleaching and treating steps being sequential and not including an intermediate washing step so as to produce a preliminarily bleached chemical pulp, washing the preliminarily bleached chemical pulp so as to produce a washed chemical pulp, and bleaching the washed chemical pulp with hydrogen peroxide.
In accordance with one embodiment of the method of the present invention, the treating step is carried out immediately prior to the bleaching step.
In accordance with another embodiment of the method of the present invention, the bleaching step is carried out immediately prior to the treating step.
In accordance with another embodiment of the method of the present step, the treating step is carried out for a period of between about 10 and 120 minutes in order to eliminate metal ions therefrom. Preferably, the metal ions are manganese ions.